1. Field of the Invention
This invention relates to a novel apparatus for placing line workers in close proximity to bundled conductors of energized high voltage power transmission lines to enable the line workers to effect repairs or maintenance thereof, while providing a means for the line worker to safely descend from the apparatus to the ground in an emergency.
2. Description of the Related Art
Electrical power is transmitted from generating facilities to consumers over long distances by means of high voltage conductors which can carry voltages ranging from 5 kilovolts to 800 kilovolts, and higher. These high voltages are ultimately stepped down by means of transformers so the electrical power can be used by consumers.
Standard high voltage alternating current transmission lines employ three conductors, each conductor carrying voltage 120.degree. out of phase to each other and referred to as a "phase".
When more than one individual conductor is used per phase, the group of conductors comprising a phase is known as "bundled conductors".
Such extra high voltage (ehv) transmission lines are serviced, repaired and maintained either in an energized or de-energized state. In order to avoid the loss of revenues resulting from shutting down or de-energizing a line to perform maintenance on it, the electrical utility industry prefers the energized state. Currently, one generally accepted method of repair, maintenance and inspection of energized transmission lines is known as "barehand work".
Barehand work is the technique of performing live maintenance on energized conductor and equipment whereby one or more line workers work directly on an energized part after having been raised and bonded to the energized conductor or equipment.
At locations away from the tower or structure, these line workers are normally placed in a position adjacent to the energized bundled conductor by an insulated boom and bucket attached to a grounded truck, by a fixed platformphysically and electrically attached to an electrically isolated hovering helicopter, or by an electrically insulated sling line suspended from a grounded crane or electrically isolated hovering helicopter.
At locations near or at the tower or structure, the line workers access the energized bundled conductors from the tower or structure by climbing down insulated ladders.
At locations away from the tower or structure, the truck boom and bucket, the fixed platform attached to the helicopter, and the sling line from the crane are required to remain in position for an extensive time which lengthens the exposure time in the electrically charged space between the energized conductors and grounded structure/tower components and increases the risk of a flashover or physical damage due to mechanical failure of the bucket truck, crane or helicopter. This risk is magnified greatly at positions of the center conductor phase on a horizontally configured three phase or bundled arrangement, usually a pair of generally parallel lines above a central lower line.
The related art discloses several systems and apparatuses for intended for use in connection with the servicing of energized transmission lines by "barehand work". U.S. Pat. No. 3,139,948 discloses a personnel carrier that is elevated by an aerial lift to support the line worker located in the carrier adjacent to an energized transmission line. The carrier has a conductive surface that is connected to the energized line and with which the line worker is in electrical contact, so that the line worker can be maintained at the same potential as the energized conductor. The carrier is insulated from the vehicle on which the aerial lift is situated and from ground. The major drawback to this system is that the vehicle and aerial lift must remain in position while the carrier is elevated to the line.
U.S. Pat. No. 4,478,312 discloses a portable inverted U-shaped cage for servicing an energized transmission line which is lowerable by helicopter from a position directly above the line thereonto. The cage consists of a horizontal frame and two baskets in which the line workers are situated, each basket connected to and hanging downwardly from the ends of the crossframe. The lower side of the crossframe includes skids or rollers to engage the line, on which the cage is aerially self-supported. Both the crossframe and cage are electrically conductive, as are the skids or rollers, as the case may be, so that the entire cage and line workers therein are electrically interconnected to the energized power line, whereby the cage and line workers become energized to the same voltage potential as that of the line. It should be noted that so long as the cage remains on the line, however, the line worker's body potential will be maintained at the potential of the line. In the event of a helicopter's mechanical failure, safety means should be provided whereby the line worker can descend to the ground, however, in this patented invention, the line worker must remain in the portable cage energized to the line potential until a replacement helicopter arrives to remove the cage from the line.
U.S. Pat. No. 4,673,059 to Kurtgis discloses a system for placing a line worker onto an energized transmission line. The line worker is positioned in a bosun's seat with a body harness that includes snap hooks or pulleys. The body harness is releasably connected to one end of a suspension means which includes an insulated link, and the other end of the suspension means is connected to a support means such as an electrically isolated helicopter or grounded crane. The helicopter or crane transports the line worker adjacent the energized line, to which the lineman connects the snap hooks or pulleys, which are electrically conductive. An emergency descent means consisting of an insulated rope and attachment device is provided so that the line worker can lower himself to the ground. The primary disadvantage of this system is that it allows only one worker at a time to be positioned directly in front of a particular section of transmission line to perform repairs or maintenance thereon and the insulated rope can become wet and hence, "conductive" which provides an electrical path to ground.
There remains a need in the art for a portable system that can be easily transported by an aerial lifting means to a bundled conductor phase, on which the system is fully self-supported, which can be moved laterally along the transmission lines without being disengaged therefrom, which permits more than one line worker at a time to perform maintenance or repairs on a particular section of energized transmission line which reduces the risk of flashover or mechanical failure due to extensive time of support equipment in a compromised position, and which provides a safety means for descending from the line. The present invention is adapted to meet these needs.